A number of adjustable socket-forming devices are described in the prior art. Such prior devices typically include two or more movable jaws and mechanisms for supporting the jaws and for varying the distance between the jaws while maintaining a rigid structure. The requirement that the devices be both adjustable and rigid has resulted in prior socket-forming devices of great complexity, such complexity being manifested by the large number of individual parts used to realize the devices, and by the intricate shapes of many of the parts. Because of such complexity, prior devices have been comparatively expensive to manufacture, and have proved vulnerable to failure, jamming and other defects.